Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 73

2.0

'1.5

|

,~1.0

0.5

,, , , , , ,| ,, , , ,,, , ,

Cr ~,~ o

0.4

",.X 4

, I ,.I .... I ' ' '

10 100 1000

Cooling rate (~

Figure 1.78 Relation between critical cooling rate and amount of alloying element
present (After Irie et al. 123)

J :S (

40o

>-

9 810~

o 760oc

T$

YS

0 10 20 30 40

Martensite (%)

Figure 1.79 Variation of yieM and tensile strength with volume per cent of martensite

for a dual-phase steel containing 0.063 wt% C and 1.29 wt% Mn annealed for 10 rain at

760 or 810"C (After Lawson et al. l~)

of the ferrite and the martensite which depends primarily on the martensite carbon

content 127 and on any solid solution strengthening of the ferrite. The martensite

carbon content depends on the annealing temperature and the initial cooling rate,

but manganese and silicon additions also influence this carbon content. 12s The

optimum combination of strength and formability is obtained by a very fine

distribution of martensite islands and a very fine grain size. '126